The traditional method for the production of ethylene from petroleum hydrocarbon is by steam thermal cracking in tubular furnace. The feeds adopted in steam thermal cracking are ethane, propane, butane, naphtha or atmospheric gas oil (AGO). The method using heavier hydrocarbon thermal cracking on solid heat carrier for producing ethylene also exists. The heat carriers generally adopted are inert solid particles such as quartz sand, coke, coke carbon, etc.
Quite a few literatures reported the methods relating the production of light olefins by petroleum hydrocarbon cracking or pyrolysis conversion with catalysts. U.S. Pat. No. 3,541,179, U.S. Pat. No. 3,647,682, DD 225,135 and SU 1,214,726 adopt the oxides supported metal catalysts, wherein the support can be SiO.sub.2.Al.sub.2 O.sub.3 or other oxides and the metal components are selected from elements of II.sub.B, V.sub.B, VII.sub.B and VIII.sub.B groups. As the supported metal possesses dehydrogenation property, it also accelerates the condensation and coking reaction during the reaction process of cracking. Hence, this type of catalysts can only be used for processing light feed stocks (&lt;220.degree. C.). Some patents adopt the composite oxides as catalysts, for example, U.S. Pat. No. 3,725,495 and U.S. Pat. No. 3,839,485 reported the catalysts comprising mainly ZrO.sub.2 and/or HfO.sub.2, and also Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, MnO.sub.2 and/or Fe.sub.2 O.sub.3 and alkali and alkaline metal oxides. Although alkali and alkaline earth metal oxide catalysts can promote the yield of ethylene, quite a lot of CO and CO.sub.2 will be formed simultaneously.
In the field of the production of light olefins by petroleum hydrocarbon cracking with solid acid catalyst, DD 152,356A adopts the amorphous SiO.sub.2.Al.sub.2 O.sub.3 catalyst for cracking various liquid hydrocarbon and hydrocarbon fractions including gasoline, kerosene, gas oil or vacuum distillates, at 600.about.800.degree. C. to produce light olefins with a C.sub.2.sup.=.about.C.sub.4.sup.= olefin yield of 40.about.50 wt % based on the feed. JP 60-224,428 disclosed a cracking catalyst using HZSM-5 as active component and alumina as matrix, for catalytic cracking a feed stock of C.sub.5.about.C.sub.25 paraffinic hydrocarbons at 600.about.750.degree. C. to obtain a C.sub.2.sup.=.about.C.sub.4.sup.= olefin yield of roughly 30 wt %. U.S. Pat. No. 3,758,403 revealed that a catalyst comprising both ZSM-5 zeolites and a large pore zeolite (e.g. X type or Y type) as active components in a ratio of 1:10.about.3:1 displayed a performance of raising gasoline octane number while increasing C.sub.3.sup.=.about.C.sub.4.sup.= olefin yield to about 10 wt %. In CN1069016A, a catalyst comprising 30.about.90 wt % SiO.sub.2, 20.about.70 wt % Al.sub.2 O.sub.3, 0.5.about.30 wt % alkali and alkaline earth metal oxides and 1.about.30 wt % faujasite can produce 17.about.27 wt % yield of C.sub.2.sup.=, 30.about.40 wt % yield of C.sub.2.sup.=.about.C.sub.4.sup.= at 650.about.900.degree. C. with the heavy hydrocarbon as feedstock. In U.S. Pat. No. 4,980,053, a catalyst comprising HZSM-5 as active component supported on kaolin matrix obtained 40 wt % yield of C.sub.3.sup.= +C.sub.4.sup.= by cracking the heavy hydrocarbon feed at 500.about.650.degree. C., but the yield of C.sub.2.sup.= was low. In CN1083092A, an acidic molecular sieve catalyst containing crosslinking pillared interlayedrectorite molecular sieve and/or rare earth pentasil high silica zeolite is used for cracking heavy hydrocarbons at 680-780 and obtains a yield of 23 wt C.sub.2 and 50 wt/C.sub.2 -C.sub.4, U.S. Pat. No. 5,380,690 and ZL CN93102783.7 revealed a catalyst using a Y type zeolite and a phosphorus and rare earth containing high silica zeolite having a structure of pentasil as active components, and this catalyst with heavy oil fractions as feed stock can increase at 500.about.600.degree. C. the gasoline octane number as well as the yield of C.sub.2.sup.=.about.C.sub.4.sup.=, mainly the yield of C.sub.3.sup.= and C.sub.4.sup.=.
The object of the present invention is to provide a novel zeolite-containing catalyst, said catalyst can attain the same level of yield of light olefins as that of steam thermal cracking at a lower reaction temperature than that of steam thermal cracking.
Another object of the present invention is to provide a method for preparing the said catalyst.
The further objects including the application of the said catalyst can be all learned from the content of the specification of the present invention including the examples.
The present invention provides a catalyst comprising a phosphorus--aluminum, phosphorus--magnesium or phosphorus--calcium containing high silica zeolite having the structure of pentasil prepared The phosphorus existing in the zeolite enables the zeolite to have higher hydrothermal activity-stability; aluminum or magnesium or calcium existing in the zeolite can adjust the acidity of the zeolite, which is favorable to the formation of C.sub.2.sup.=. During the catalytic pyrolysis process in the presence of an acidic catalyst, hydrocarbon cracking reaction is carried out based on the carbonium ion reaction mechanism, hence the light olefins produced are mainly C.sub.3.sup.= and C.sub.4.sup.=. C.sub.2.sup.= is commonly formed by free radical reaction mechanism in thermal cracking hydrocarbons, therefore acidic catalyst is unfavorable for the formation of C.sub.2.sup.=. The acidity of the zeolite of the present invention is appropriately adjusted, thus the catalyst of the invention not only decreases the activation energy of the hydrocarbon cracking reaction, and consequently the reaction can proceed at a lower temperature than tat of steam thermal cracking, but also increases the yield of C.sub.2.sup.=.
The present invention provides a catalyst comprising 10.about.70 wt % (based on the weight of the catalyst) of clay, 5.about.85 wt % of inorganic oxides, and 1.about.50 wt % of zeolite, said zeolite is a of 0.about.25 wt % of Y type zeolite and 75.about.100 wt % phosphorus--aluminum, phosphorus--magnesium or phosphorus--calcium containing high silica zeolite having a structure of pentasil.
Said clay in the catalyst of the invention can be all kind of clays usually used as a carrier for cracking catalyst, either natural or synthesized such as kaolin and kaolin polyhydrate, and may optionally be subjected to various chemical and/or physical treatment.
Said inorganic oxides in the catalyst of the invention are selected from amorphous SiO.sub.2.Al.sub.2 O.sub.3, Al.sub.2 O.sub.3 and/or SiO.sub.2.
Said Y type zeolite in the catalyst of the invention may be a rare earth metal ion exchanged Y (REY) zeolite with a content of rare earth not less than 14 wt % (calculated as RE.sub.2 O.sub.3), and may also be a stabilized high silica Y type zeolite with a higher silica/alumina mole ratio prepared by various chemical and/or physical method, such as hydrothermal method, acid treatment, framework silicon enriching method, or SiCl.sub.4 treatment method.
Said phosphorus and aluminum or phosphorus and magnesium or phosphorus and calcium containing high silica zeolite having a structure of pentasil in the catalyst of the invention is a kind of high silica zeolite containing 2-8 wt % (based on the weight of zeolite) of phosphorus (calculated as P.sub.2 O.sub.5) and 0.3.about.3 wt % of aluminum or magnesium or calcium (calculated as oxide), such as ZSM-5, ZSM-8 or ZSM-11 type zeolite, having a silica/alumina mole ratio of preferably 15.about.60. The said phosphorus and aluminum or phosphorus and magnesium or phosphorus and calcium containing high silica zeolite having a structure of pentasil may also contain 0.3.about.3 wt % (based on the weight of zeolite) of nickel (calculated as oxide).
The preparation method of the catalyst of the invention is as follows: mixing the precursor of the inorganic oxides with clay according to a predetermined ratio, adding de-cationized water to obtain a slurry with a solid content of 10.about.50 wt %, adjusting and maintaining the pH value of the slurry to 2.about.4 using an inorganic acid such as hydrochloric acid, nitric acid, phosphoric acid or sulfuric acid, after aging statically at 20.about.80.degree. C. for 0.about.2 hrs adding into it a pre-calculated amount of zeolite, homogenizing, spray drying, washing off the free sodium ions and drying.
Said precursor of the inorganic oxides is selected from aluminum sol, pseudo-boehmite, silica sol or its mixture, and silica-alumina sol or gel.
Said clay can be all kind of clays usually used as a carrier for cracking catalyst, either natural or synthesized, such as kaolin, kaolin polyhydrate, and may optionally be subjected to various chemical and/or physical treatment.
Said inorganic acids are selected from hydrochloric acid, nitric acid, phosphoric acid or sulfuric acid.
Said zeolite is a phosphorus and aluminum or phosphorus and magnesium or phosphorus and calcium containing high silica zeolite having a structure of pentasil or a mixture of said high silica zeolite and a Y type zeolite, wherein the former is a kind of high silica zeolite containing 2.about.8 wt % (based on the weight of zeolite) of phosphorus (calculated as P.sub.2 O.sub.5) and 0.3.about.3 wt % of aluminum or magnesium or calcium (calculated as oxide) such as ZSM-5, ZSM-8 or ZSM-11 type zeolite, having a silica/alumina mole ratio of preferably 15.about.60.
The said high silica zeolite may also contain 0.3.about.3% (based on the weight of zeolite) of nickel (calculated as oxide).
The preparation method of the said zeolite is as follows: mixing homogeneously the high silica zeolite having a structure of pentasil with an aqueous solution containing phosphorus and aluminum or phosphorus and magnesium or calcium compounds according to a water/solid weight ratio of 1.about.3:1, impregnating for 0.5.about.4 hrs, drying at 100.about.120.degree. C. and then calcining at 450.about.650.degree. C. for 1.about.4 hrs. Nickel on the said high silica zeolite can also be introduced by mixing nickel compound together with phosphorus and aluminum or phosphorus and magnesium or phosphorus and calcium compounds in an aqueous solution and impregnating said high silica zeolite in said aqueous solution. The said high silica zeolites having a structure of pentasil are the type of high silica zeolites of ZSM-5, ZSM-8 or ZSM-11 type having a silica/alumina mole ratio of preferably 15.about.60. The said phosphorus compound containing aqueous solution is preferably the aqueous solution of phosphoric acid, the aluminum or magnesium or calcium compound containing aqueous solution can be the aqueous solution of the nitrate, hydrochloride and sulfate of these elements, preferably the aqueous solution of nitrate or hydrochloride. The said nickel compounds can be nitrate, hydrochloride, or sulfate.
The Y type zeolite in the catalyst of the invention can be the rare earth metal ion exchanged REY zeolite having a rare earth metal content of not less than 14 wt % (calculated as RE.sub.2 O.sub.3) based on the weight of the zeolite, and may also be a stabilized high silica Y type zeolite having a higher silica/alumina mole ratio prepared by various chemical method and/or physical method, such as hydrothermal method, acid treatment method, framework silica enriching method, or SiCl.sub.4 treatment method.
The catalyst provided by the present invention can attain the same level of yield of light olefins as that of steam thermal cracking reaction at a lower reaction temperature than that of steam thermal cracking reaction. The said catalyst displays the excellent hydrothermal activity-stability as well as the yields of light olefins, especially the yields of C.sub.2.sup.= and C.sub.3.sup.=. For example, the said catalyst can achieve a yield of 20 wt % (based on the feed) of C.sub.2.sup.=, with the total yields of C.sub.2.sup.=.about.C.sub.4.sup.= being up to 54 wt % under the reaction conditions of 680.degree. C., a catalyst/oil weight ratio of 10 and WHSV (weight hour space velocity) of 10 hrs.sup.-1, using VGO (vacuum gas oil) as feedstock.
The present invention well be further described with reference to the following examples, which, however, shall not be construed to limit the scope of the present invention.
The phosphorus and aluminum or phosphorus and magnesium or phosphorus and calcium containing high silica zeolite, which may optionally contain nickel, having a structure of pentasil used in the examples is prepared as follows: ion-exchanging the commercial product of ZSM-5 zeolite having a silica/alumina mole ratio of 25 with ammonium nitrate solution according to a weight ratio of zeolite (anhydrous basis): ammonium nitrate:de-cationized water=1:1:20 at 90.degree. C. for 2 hrs, after filtering and leaching the resultant was ion-exchanged once more to obtain ammonium type of ZSM-5 with the sodium content (calculated as Na.sub.2 O) of less than 0.15 wt % based on the weight of the zeolite, mixing said homogeneously ammonium type of ZSM-5 with a H.sub.3 PO.sub.4, de-cationized water and AlCl.sub.3 or MgCl.sub.2 or CaCl.sub.2 containing solution, in which Ni(NO.sub.3).sub.2 may optionally added according to a liquid /solid weight ratio of 2;1, followed by stirring and impregnating for 2 hrs, drying at 120.degree. C., and then calcining at 550.degree. C. for 2 hrs. The resulting ZSM-5 prepared by the above procedure containing 4.9 wt% of P.sub.2 O.sub.5 and 0.9 wt % of Al.sub.2 O.sub.3 is marked as P.Al--Z zeolite, the one containing 5.0 wt % of P.sub.2 O.sub.5 and 1.4 wt % of MgO is marked as P.Mg--Z zeolite, the one containing 4.9 wt % P.sub.2 O.sub.5 and 2.0 wt % of CaO is marked as P.Ca--Z zeolite. The one containing 5.0 wt % of P.sub.2 O.sub.5, 1.4 wt % of MgO and 1.2 wt % of NiO is marked as P.Mg.Ni--Z.